Optical switch and matrix optical switch

ABSTRACT

The invention provides an optical switch including a substrate which has conductivity or semiconductivity, an optical waveguide layer which is formed on the substrate, and a control electrode which is formed on the optical waveguide layer. The optical waveguide layer includes an incident-side channel waveguide to which a light signal is incident and plural outgoing-side channel waveguides branched from the incident-side channel waveguide. The control electrode forms a reflection plane reflecting the incident light signal near a crossover portion of the plural outgoing-side channel waveguides by applying voltage to the optical waveguide layer with the substrate to control a refractive index of the optical waveguide layer, and switches propagation paths of the light signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical switch and a matrix opticalswitch, and particularly to an optical switch which switches opticalpaths of a light signal propagating through a channel waveguide and amatrix optical switch in which multiple of the optical switches arearranged in a matrix.

2. Description of the Related Art

Optical communication networks are developing, from: point-to-pointoptical communication, in which nodes are connected individually;through optical communication, in which Add-Drop Multiplexing isperformed between points; and further to optical communication in whichplural nodes are directly connected, without converting a light signalinto an electric signal. Therefore, development of various opticalcomponents necessary for the above optical communication becomesimportant, such as optical splitter/couplers, optical multiplexers,optical demultiplexers, optical switches, and the like. Among these,matrix optical switches are some of the most important components, beingused for switching light signal paths among plural optical fibers inresponse to demand, or for switching light signal paths in order tosecure diversion paths in the case of a network failure.

The optical switches include a bulk type of optical switches in whichprisms, mirrors, fibers, and the like are mechanically moved to switchthe light signal paths, and optical waveguide types of optical switch.The bulk type of optical switch has the advantage that wavelengthdependence is small and loss is relatively low. However, there arevarious problems with the bulk type of optical switch such as: lowswitching speed; unsuitability for formation into matrices, due to thedifficulty of miniaturization; unsuitability for mass production,because the assembly and adjustment process is complicated; expense; andthe like. On the other hand, because the optical waveguide type ofoptical switch is significantly superior to the bulk type of opticalswitch in terms of switching speed, miniaturization, integration, massproduction, and the like, the optical waveguide type of optical switchis being avidly investigated.

Optical waveguide type of matrix optical switches can be divided intotwo main modes. In the first mode, the paths of propagating lightsignals are switched by connecting a branching type of channel waveguidebetween input and output ports, and optical switches or optical gates,operated by predetermined principles are arranged at branching points.In the second mode, a light deflector is provided between the input andoutput ports to deflect the incident light from input ports towardoutput ports.

Currently, the matrix optical switch of the first mode is being mostactively investigated, because of its design flexibility and smalloptical loss. Generally in the first mode of matrix optical switch, achannel waveguide is formed in a thin film made of LiNbO₃, compoundsemiconductor, quartz, polymer, or the like. At crossover portions oneach path there is provided either: an optical switch electricallycontrolling the direction of travel of the light; or an optical gate,electrically controlling the direction of travel of the light by openingand closing.

The operating principles of the optical switch include: a method ofcontrolling the light signal path by applying an electric field to adirectional coupler in which two optical waveguides are arranged closeto each other; a Mach-Zehnder type of method in which an input lightbeam is separated into two light beams by a directional coupler, phasedifference is provided between the light beams passing through therespective paths by means of a refractive index generated by an electricfield, and output ends are switched by controlling interference statesusing a directional coupler positioned on an exit side; a method ofswitching light signal paths by controlling interference between opticalmodes at X-crossover portions; a so-called digital type of method inwhich light signal paths are switched by controlling a fielddistribution in transverse direction of the optical mode, by means of arefractive index generated by an electric field at Y-branching portionsor at asymmetrical X-crossover portions; and a method of switching lightsignal paths in which total reflection or Bragg reflection is made tooccur by providing electrodes at X-crossover portions to control therefractive index (Japanese Patent Laid-Open (JP-A) No. 7-318986 andJapanese Patent Publication (JP-B) No. 6-5350).

Among the above, the digital type of optical switch is superior inoperational tolerance. In the digital type of optical switch, afterlight signal paths are switched with a predetermined voltage or current,this state can be maintained, and plural operation points are notgenerated, even if a voltage or current greater than predetermined isapplied thereto. Further, advantages such as a digital type of opticalswitch independent of the wave polarization being possible, small degreeof wavelength dependence, and the like, make the digital type of opticalswitch particularly noteworthy among optical switches.

However, in the conventional digital type optical switch, when comparedwith other types of optical switches, there are the problems ofincreased drive voltage (or increased drive current) and increasedelectrode length.

FIG. 18 shows a standard Y-branching type of structure for a digitaltype of optical switch. In the optical switch having the structure shownin FIG. 18, electrodes 2 constituting an optical control portion areprovided at the branching portion of a Y-branching type of channelwaveguide 1. An acute angle portion of the crossover portion of thechannel waveguide 1 has a shape with a crossing angle less than 10 inwhich the channel separation gradually narrows to become zero. Becauseof this, with a patterning process of photolithography, it is difficultto produce an ideal shape due to resolution limitations. Therefore,usually it is necessary to form a shape where the tip end is not sharpand the distance between the channels is not less than 1.5 μm, as in theacute angle portion 3 depicted in FIG. 19. The shift from the idealshape greatly affects the degree of loss and crosstalk, because theoptical control portion is located at the branching portion on thedownstream side of the crossover portion in the light propagationdirection.

For example, with an open angle of the Y-branching of 0.50 and therefractive index of one of the branched waveguides being decreased byabout 0.0008 due to the electro-optic effect, as long as the acute angleportion has the ideal shape shown in FIG. 18, crosstalk can bedecreased. In other words the difference in light quantity betweenoutgoing ports can be made greater or equal to 20 dB when light from anincident port is guided to the outgoing ports or other. On the otherhand, if the shape is not sharp, as shown in FIG. 19, the difference inlight quantity between the outgoing ports is degraded to about 12 dB. Inorder to increase the difference in light quantity between the outgoingports to 20 dB, a larger change in refractive index is required. Thatis, in a Y-branching type of digital optical switch, there is a problemthat a drive voltage or drive current increases.

Further, because an electrode is formed on the channel waveguide havinga width of a few micrometers, production errors during photolithographyeasily occur, and symmetry of switching characteristics is easily lost.

In an X-crossover type of total reflection optical switch, as shown inFIG. 20, digital type of operation can also be performed. An X-crossovertype of total reflection optical switch is suitable for high-speedresponse because electrode length can be more easily shortened whencompared with other types. In addition, because the optical controlportion is located within the crossover portion, unlike in a Y-branchingtype of optical switch, the X-crossover type of total reflection opticalswitch is less sensitive to the above-described production limitations.In the total reflection optical switch, an incident light beam 4propagates rectilinearly when the refractive index of a channelwaveguide 1 is uniform. When a voltage is applied to an electrode 2 todecrease the refractive index of a reflection plane 5 to the refractiveindex necessary for total reflection, the incident light beam 4 istotally reflected in the reflection plane 5. A crossing angle 6 of thechannel waveguide 1 and an angle formed by the incident light beam 4 andthe reflection plane 5 (reflection supplementary angle 7) are determinedby the degree of decrease in refractive index of the reflection plane 5.The degree of decrease in refractive index becomes smaller, i.e., adrive voltage or drive current is lowered, as the crossing angle 6 andthe reflection supplementary angle 7 are decreased. The crossing angle 6can usually be decreased to about 0.5°.

However, in reality, when the crossing angle of the X-shaped crossoverportion is decreased to the range of 1° to 2°, there is a problem thatdrive voltage increases or crosstalk increases.

For example, an X-crossover type of total reflection optical switch inwhich a channel waveguide having a width of 4 μm with a crossing angleof 1.0° is formed by diffusing Ti into LiNbO₃ is described in C. S.Tsai, et al., J. Quantum Electronics, (1978) 513. In the totalreflection optical switch, it is expected that a response speed of 5.9GHz can be obtained by providing electrodes with a gap of 4 μm while ataper type channel waveguide having a maximum width of 40 μm is providedin order to decrease crosstalk at the crossover portion. However, thedrive voltage becomes as large as 50V.

The X-crossover type of total reflection optical switch in which anepitaxial PLZT thin film waveguide layer is grown on a sapphiresubstrate, which is an insulating material, to form a channel waveguidehaving a crossing angle of 2.0° and a width of 20 μm is described in K.Wasa, et al., J. Lightwave Technology, (1984) 710. In the totalreflection optical switch, the X-crossover type of total reflectionoptical switch is formed by providing electrodes with a gap of 4 μm onthe channel waveguide, and a response speed of 1 GHz is obtained at4.7V. However, crosstalk is still as large as 12 dB.

An X-crossover type of total reflection optical switch having a crossingangle of 4.0° and a width of 14 μm, which is formed by using a polymerwaveguide, is described in T. Ichigi et al., OFC 2002, 187. In the totalreflection optical switch, crosstalk is decreased such that thedifference in light quantity between outgoing ports is 30 dB or greater.However, drive electric power as large as 100 mW is required because thethermo-optic effect is utilized, and the response speed is only about 1ms. That is, when the crossing angle of the X-crossover portion isrelatively large, while crosstalk is decreased, electric powerconsumption is increased. In an optical switch made of polymer, sincethe thermo-optic effect is utilized, the merit of a high-speed responseof the total reflection optical switch cannot be utilized.

As described above, in a total reflection optical switch operated bycontrolling the refractive index, a digital type of response can beobtained, and the total reflection optical switch is suitable for ahigh-speed response. However, with the total reflection type it isdifficult to obtain an optical switch in which both the drive voltage ordrive current is low and crosstalk is low.

SUMMARY OF THE INVENTION

In view of the foregoing, the invention provides an optical switch thatis of a total reflection optical switch in which a digital response andminiaturization can be realized, and in which a drive voltage or a drivecurrent is low and crosstalk is decreased, and a matrix optical switchin which the multiplicity of optical switches are arrayed in a matrix.

An optical switch of a first aspect of the invention includes asubstrate which has conductivity or semiconductivity, an opticalwaveguide layer which is formed on the substrate, the optical waveguidelayer including an incident-side channel waveguide on which a lightsignal is incident and plural outgoing-side channel waveguides branchingfrom the incident-side channel waveguide, and a control electrode whichis formed on the optical waveguide layer, the control electrode forming,near a crossover portion of the plural outgoing-side channel waveguides,a reflection plane which reflects the incident light signal by applyingwith the substrate a voltage to the optical waveguide layer to controlthe refractive index of the optical waveguide layer, and the controlelectrode switching propagation paths of the light signal.

In the optical switch of the first aspect, the control electrode and thesubstrate apply voltage to the optical waveguide layer, and a reflectionplane is formed near the crossover portion of the plural outgoing-sidechannel waveguides branched from the incident-side channel waveguide.Therefore, a digital type total reflection optical switch can be formedand an angle (reflection supplementary angle) formed by the centerlineof the outgoing-side channel waveguide and the reflection plane can bedecreased. For example, even if the crossing angle of the Y-branchingoutgoing-side channel waveguide is equal to the crossing angle of theX-crossover type of total reflection optical switch, the reflectionsupplementary angle of the optical switch of the first aspect can becomea half of the reflection supplementary angle of the X-crossover type oftotal reflection optical switch. Therefore, a degree of the decrease inrefractive index can be made smaller at the reflection plane and a drivevoltage or a drive current can be decreased.

An optical switch of a second aspect of the invention includes asubstrate which has conductivity or semiconductivity, an opticalwaveguide layer which is formed on the substrate, the optical waveguidelayer including an incident-side channel waveguide on which a lightsignal is incident and plural outgoing-side channel waveguides branchingfrom the incident-side channel waveguide, the optical waveguide layerhaving a grooved portion formed between adjacent outgoing-side channelwaveguides, and a control electrode which is formed on the opticalwaveguide layer, the control electrode forming, near a crossover portionof the plural outgoing-side channel waveguides, a reflection plane whichreflects the incident light signal by applying a voltage with thesubstrate to the optical waveguide layer to control a refractive indexof the optical waveguide layer, the reflection plane being contiguous toan interface between the optical waveguide layer and the groovedportion, and the control electrode switching propagation paths of thelight signal.

A matrix optical switch of a third aspect of the invention includes asubstrate which has conductivity or semiconductivity and plural opticalswitch units which are arranged in a matrix on the substrate, whereineach of the plural optical switch units includes an optical waveguidelayer which is formed on the substrate, the optical waveguide layerincluding an incident-side channel waveguide on which a light signal isincident and plural outgoing-side channel waveguides branching from theincident-side channel waveguide, and a control electrode which is formedon the optical waveguide layer, the control electrode forming, near acrossover portion of the plural outgoing-side channel waveguides, areflection plane which reflects the incident light signal by applying avoltage with the substrate to the optical waveguide layer to control therefractive index of the optical waveguide layer, and the controlelectrode switching propagation paths of the light signal.

A matrix optical switch of a fourth aspect of the invention includes asubstrate which has conductivity or semiconductivity and plural opticalswitch units which are arranged in a matrix on the substrate, whereineach of the plural optical switch units includes an optical waveguidelayer which is formed on the substrate, the optical waveguide layerincluding an incident-side channel waveguide on which a light signal isincident and plural outgoing-side channel waveguides branching from theincident-side channel waveguide, the optical waveguide layer having agrooved portion formed between adjacent outgoing-side channelwaveguides, and a control electrode which is formed on the opticalwaveguide layer, the control electrode forming, near a crossover portionof the plural outgoing-side channel waveguides, a reflection planereflecting the incident light signal by applying with the substrate avoltage to the optical waveguide layer to control the refractive indexof the optical waveguide layer, the reflection plane being contiguous toan interface between the optical waveguide layer and the groovedportion, and the control electrode switching propagation paths of thelight signal.

As described above, in accordance with the invention, a total reflectionoptical switch, in which a digital response and miniaturization can berealized, has an effect that crosstalk is decreased while a drivevoltage or a drive current is decreased.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiment of the present invention will be described indetail based on the following figures, wherein:

FIG. 1 is a plan view showing a structure of a Y-branching type of 1×2optical switch according to an embodiment of the invention;

FIG. 2 is a sectional view of the 1×2 optical switch taken on line A-A′of FIG. 1;

FIG. 3 is a plan view showing a waveguide structure of the 1×2 opticalswitch of FIG. 1;

FIGS. 4A to 4C are a plan view for explaining operation of the 1×2optical switch of FIG. 1;

FIG. 5 is a sectional view along a light propagation direction of the1×2 optical switch of FIG. 1;

FIG. 6 is a graph showing calculation result of a change in refractiveindex necessary to obtain a 20-dB crosstalk when a maximum width of ataper portion is changed;

FIG. 7 is a graph showing calculation result of a change in refractiveindex necessary to obtain a 20-dB crosstalk when a taper length ischanged;

FIG. 8 is a graph showing calculation result of a change in refractiveindex necessary to obtain a 20-dB crosstalk when a length of a linearportion is changed;

FIG. 9 is a graph showing a relationship between drive voltage andcrosstalk in the 1×2 optical switch of Example 1;

FIG. 10 is a plan view showing another structure of the Y-branching typeof 1×2 optical switch of the invention;

FIG. 11 is a plan view showing a structure of the 1×2 optical switch ofExample 2;

FIG. 12 is a graph showing a relationship between drive voltage andcrosstalk in the 1×2 optical switch of Example 2;

FIG. 13 is a graph showing a relationship between drive voltage andcrosstalk in the 1×2 optical switch of Example 3;

FIG. 14 is a plan view showing a schematic configuration of a 1×8optical switch of Example 4;

FIG. 15 is a plan view showing a structure of the 1×2 optical switch ofExample 6;

FIG. 16 is a sectional view of the 1×2 optical switch taken on line B-B′of FIG. 15;

FIG. 17 is a plan view showing a structure of the Y-branching type of1×2 optical switch formed by a LiNbO₃ waveguide;

FIG. 18 is a plan view showing a schematic configuration of theconventional Y-branching type of 1×2 optical switch;

FIG. 19 is a partially expanded view showing a structure of a branchingportion in the optical switch of FIG. 18; and

FIG. 20 is a plan view showing a schematic configuration of anX-crossover type of total reflection optical switch.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the accompanying drawings, an embodiment of the inventionwill be described in detail below.

(Structure of Optical Switch)

FIG. 1 is a plan view of a Y-branching type of optical switch accordingto an embodiment of the invention, and FIG. 2 is a sectional view of theoptical switch taken on line A-A′ of FIG. 1.

As shown in FIGS. 1 and 2, an optical switch 10 includes a conductivesubstrate 12 which becomes a lower electrode, an optical waveguide layer16 in which a channel waveguide 24 is formed, a buffer layer 14, and anupper electrode 26. The signal light propagates through the channelwaveguide 24. The buffer layer 14 has the refractive index lower thanthat of the optical waveguide layer 16. The buffer layer 14 prevents thelight propagating through the optical waveguide layer 16 from exuding tothe conductive substrate 12. A voltage is applied to the opticalwaveguide layer 16 with the lower electrode and the upper electrode 26.

The optical waveguide layer 16 is laminated on the conductive substrate12 through the buffer layer 14, and the upper electrode 26 is formed onthe optical waveguide layer 16. It is also possible that a claddinglayer having the refractive index lower than that of the opticalwaveguide layer 16 is provided between the optical waveguide layer 16and the upper electrode 26. In the optical switch 10, the refractiveindex of the optical waveguide layer 16 is partially decreased byapplying a voltage between the upper and lower electrodes, which allowsa reflection plane to be formed along an edge of the upper electrode 26in the optical waveguide layer 16 to switch the propagation path of thesignal light.

The optical switch 10 is a 1×2 optical switch including one incidentport 18 and two outgoing ports 20 and 22. The channel waveguide 24branched in the Y-shape is formed in the optical waveguide layer 16. TheY-shaped channel waveguide 24 includes a channel waveguide 24A andchannel waveguides 24B and 24C branched from the channel waveguide 24A.A light signal is incident to the channel waveguide 24A from theincident port 18. The channel waveguides 24B and 24C output the lightsignal to the outgoing ports 20 and 22, respectively.

As shown in FIG. 3, a taper portion 34A is formed in a tapered shape onthe outgoing side of the channel waveguide 24A so as to extend towardthe propagation direction of the light signal. A linear portion 36A iscontinuously added to the taper portion 34A. Taper portions 34B and 34Care formed in a reversely tapered shape on the incident sides of thechannel waveguides 24B and 24C so as to extend toward the oppositedirection to the propagation direction of the light signal. Linearportions 36B and 36C are continuously added to taper portions 34B and34C respectively. Therefore, a Y-crossover portion 30 in which thesechannel waveguides are crossed with each other is formed in a widenedshape by the taper portions 34A, 34B, and 34C and the linear portions36A, 36B, and 36C, and the crosstalk is prevented. It is possible thatthe linear portions 36A, 36B, and 36C may not be provided, ifappropriate, in accordance with a degree of crossover among the taperportions 34A, 34B, and 34C.

FIG. 4A shows each of centerlines of the channel waveguides 24A, 24B,and 24C. A crossing angle 38 is one which is formed at the Y crossoverportion 30 by the centerline of the channel waveguide 24B and thecenterline of the channel waveguides 24C. The crossing angle 38 has thesame value in both an X-crossover and an Y-crossover. In order todecrease drive voltage (or drive current) to the range from about 5V toabout 20V, it is preferable that the crossing angle 38 ranges from 0.5°to 2.0°.

As shown in FIG. 1, upper electrodes 26A, 26B, and 26C are disposed as acontrol electrode on the Y-crossover portion 30. Each of the upperelectrodes 26A, 26B, and 26C is arranged in a predetermined layout sothat signal light 44 incident from the incident port 18 is totallyreflected from the reflection plane formed along the edge of eachelectrode to be outputted to the outgoing port 20 or 22.

For example, when the signal light is coupled to the outgoing port 20,as shown in FIG. 4B, a voltage is applied between the conductivesubstrate 12 and the upper electrodes 26B and 26C to form a reflectionplane 42B along the edges of the upper electrodes 26B and 26C on thewaveguide side. The signal light 44 incident from the incident port 18propagates along the centerline of the channel waveguide 24A to betotally reflected from the reflection plane 42B, and the signal light 44is outputted to the outgoing port 20 along the centerline of the channelwaveguide 24B.

The condition of the total reflection is that the angle formed by thecenterline of the channel waveguide 24A and the reflection plane 42Bbecomes equal to the angle formed by the centerline of the channelwaveguide 24B and the reflection plane 42B. The angle of the totalreflection is a total reflection supplementary angle 40.

Similarly, when the signal light is coupled to the outgoing port 22, asshown in FIG. 4C, a voltage is applied between the conductive substrate12 and the upper electrodes 26A and 26C to form a reflection plane 42Aalong the edges of the upper electrodes 26A and 26C on the waveguideside. The signal light 44 incident from the incident port 18 propagatesalong the centerline of the channel waveguide 24A to be totallyreflected from the reflection plane 42A, and the signal light 44 isoutputted to the outgoing port 22 along the centerline of the channelwaveguide 24C.

The upper electrode 26C is formed in a triangle having two sides thatare the edges along the reflection plane 42A and along the reflectionplane 42B, respectively. The upper electrode 26C is used as a commonelectrode to which a voltage is applied when the signal light 44 iscoupled to either of the outgoing port 20 or 22. Instead of theformation of the upper electrode 26C, as shown in FIG. 10, it is alsopossible that a part of the channel waveguide corresponding to the upperelectrode 26C is removed by etching or the like to form a groove portion(trench) 46. In an optical switch 50, the switching can be performedonly by applying the voltage to the upper electrodes formed at twopositions, which simplifies the control of the refractive index.

Although the total reflection supplementary angle is a half of thecrossing angle in an X-crossover, the total reflection supplementaryangle 40 can be set to a quarter of the crossing angle 38 in the opticalswitch having the above-described structure because the optical switchhas a Y-crossover. Therefore, the decrease in refractive index can besuppressed to a small amount in the reflection plane 42B, which allowsthe drive voltage or the drive current to be decreased. The reflectionplane is not formed on the branched channel waveguides 24B and 24C, butthe reflection plane is formed by providing the electrode on theY-crossover portion 30, so that the reflection plane is hardly affectedby the shape of the branching portion or a production error of the upperelectrode 26, and the crosstalk is decreased when compared with theconventional Y-branching optical switch. In particular, the upperelectrode 26 is formed on the Y-crossover portion 30 having a large areain the embodiment, so that the degradation of the switchingcharacteristics is hardly generated by the production error of the upperelectrode 26.

(Structure of Taper Portion)

The preferable structures of the taper portion and the linear portion inthe channel waveguide 24 will be described below.

It is preferable that the maximum width of the taper portion ranges from5 μm to 50 μm. The maximum width of the taper portion is appropriatelyoptimized in accordance with the refractive index of the opticalwaveguide layer, the difference in refractive index between the opticalwaveguide layer and the cladding layer, the width of the channelwaveguide, the crossing angle, and the like.

FIG. 6 shows calculation result of the change in refractive indexnecessary to obtain a 20-dB crosstalk when the maximum width of thetaper portion is variously changed on the following conditions withrespect to the optical switch according to the embodiment.

Refractive index of optical waveguide layer: 2.43

Difference in refractive index between channel optical waveguide andcladding layer: 0.3% or 0.4%

Width of channel waveguide: 4 μm

Taper length (L₁ of FIG. 3): 700 μm

Y-crossing angle: 0.5°

As can be seen from FIG. 6, when the maximum width of the taper portionis not more than the range from 10 μm to 18 μm, a large and rapid changein refractive index is required. Therefore, in order not to generate thedegradation of the switching characteristics, it is desirable that themaximum width of the taper portion is not lower than at least 10 μm.Since a large change in refractive index is also required when themaximum width of the taper portion is not less than the range from 24 μmto 26 μm, it is desirable that the maximum width of the taper portion isnot more than at least 26 μm.

It is preferable that the taper length ranges from 200 μm to 2000 μm.The taper length is appropriately optimized in accordance with therefractive index of the optical waveguide layer, the difference inrefractive index between the optical waveguide layer and the claddinglayer, the width of the channel waveguide, the crossing angle, and thelike. When the taper length is shorter than 200 μm, degradation of theswitching characteristics or increase in radiation loss is generated. Onthe other hand, when the taper length is longer than 2000 μm, theswitching characteristics are also gradually degraded. Further, when thetaper length is increased, there is a problem that the device becomeslarge or the like.

FIG. 7 shows the calculation result of the change in refractive indexnecessary to obtain a 20-dB crosstalk when the taper length is variouslychanged on the following conditions with respect to the optical switchaccording to the embodiment.

Refractive index of optical waveguide layer: 2.43

Difference in refractive index between channel optical waveguide andcladding layer: 0.3% or 0.4%

Width of channel waveguide: 4 μm

Maximum width of taper portion: 22 μm

Y-crossing angle: 0.5°

As can be seen from FIG. 7, when the taper length is not more than therange from 400 μm to 600 μm, a large and rapid change in refractiveindex is required. Therefore, in order not to generate degradation ofthe switching characteristics, it is desirable that the taper length isnot less than 600 μm. Although a somewhat large change in refractiveindex is also required when the taper length is not less than the rangefrom 700 μm to 800 μm, the switching characteristics are not remarkablydegraded. Therefore, in view of miniaturization of the device, it isdesirable that the taper length is not more than 1000 μm.

It is also possible that the taper length on the incident-side of thechannel waveguide is different from the taper length on theoutgoing-side of the channel waveguide. The switching characteristicscan be optimized by the different taper lengths.

The length of the linear portion (L₂ of FIG. 3) connected to the taperportion can be mainly determined by the relationship with the taperlength. It is preferable that the length of the linear portion ranges upto 1600 μm. The length of the linear portion is appropriately optimizedin accordance with the refractive index of the optical waveguide layer,the difference in refractive index between the optical waveguide layerand the cladding layer, the width of the channel waveguide, the crossingangle, and the like. When the length of the linear portion is shorterthan 200 μm, the tendency of the switching characteristics degradationis generated. On the other hand, when the length of the linear portionis longer than 1600 μm, the switching characteristics are also degraded.Further, there is a problem that the device becomes large or the like.

FIG. 8 shows the calculation result of the change in refractive indexnecessary to obtain a 20-dB crosstalk when the length of the linearportion is variously changed on the following conditions with respect tothe optical switch according to the embodiment.

Refractive index of optical waveguide layer: 2.43

Difference in refractive index between channel optical waveguide andcladding layer: 0.4%

Width of channel waveguide: 4 μm

Maximum width of taper portion: 22 μm

Taper length L₁: 700 μm

Y-crossing angle: 0.5°

As can be seen from FIG. 8, it is desirable that the length of thelinear portion ranges from 200 μm to 800 μm.

(Material of Each Layer)

Each layer constituting the optical switch will be described below.

When light is introduced into the optical waveguide layer provided onthe substrate, generally, part of total light intensity exudes to theconductive substrate having lower transparency, the exuded component isabsorbed into the conductive substrate, and propagation loss isgenerated in accordance with the light propagation. However, as shown inFIG. 5, when a portion of the thickness where exuding occurs is replacedwith the buffer layer 14, the exuded component is not absorbed into theconductive substrate 12 and the propagation loss can be decreased. Inorder that the buffer layer 14 functions as an isolation layer betweenthe optical waveguide layer 16 and the conductive substrate 12, it isnecessary that the refractive index of the material of the buffer layer14 is smaller than that of the optical waveguide layer 16.

In order that the light propagation loss arising from scattering bygrain boundaries existing on the surface of the optical waveguide layer16 or in the optical waveguide layer 16 is decreased to a practicallevel, it is essential that the material of the buffer layer 14 hold anepitaxial relationship with the material of the conductive substrate orthe optical waveguide layer. It is desirable that the material of theoptical waveguide layer 16 has a higher electro-optic coefficient, andit is desirable that the material of the conductive substrate 12 haslower resistivity. In order to decrease the propagation loss to a valuenot more than 1 dB/cm, it is necessary that a film thickness ratio ofthe buffer layer 14 to the optical waveguide layer 16 should be not lessthan at least 0.1. When operation is predicated in a TE₀ single mode, itis proper to set the film thickness ratio not less than 0.5.

Since the upper electrode 26 is provided on the optical waveguide layer16, the voltage applied between the upper and lower electrodes isdistributed in accordance with each capacity of the optical waveguidelayer 16 and the buffer layer 14, which causes decrease in effectivevoltage that can be applied to the optical waveguide layer 16, when thebuffer layer 14 exists between the conductive substrate 12 and theoptical waveguide layer 16. However, a higher effective voltage can beapplied to the optical waveguide layer 16 by using the buffer layer 14which has a constant film thickness and high dielectric constant.

In the optical switch having the above-describe structure, LiNbO₃, acompound semiconductor, quartz, polymer, and the like can be utilized asthe waveguide material. From the viewpoints of high-speed response, lowpower consumption, low light loss, and miniaturization, it is desirable,in particular, to use the following materials.

A conductive or semiconductive single crystal substrate or a substratein which an epitaxial thin film or a conductive or semiconductive thinfilm having a single orientation is provided on the surface can be usedas the conductive substrate 12 which is of a lower electrode. An oxide,such as SrTiO₃ doped with Nb, La, or the like, Al-doped ZnO, In₂O₃,RuO₂, BaPbO₃, SrRuO₃, YBa₂CU₃O_(7-X), SrVO₃, LaNiO₃,La_(0.5)Sr_(0.5)CoO₃, ZnGa₂O₄, CdGa₂O₄, CdGa₂O₄, Mg₂TiO₄, and MgTi₂O₄; asingle semiconductor such as Si, Ge, and diamond; a III-V compoundsemiconductor such as AlAs, AlSb, AlP, GaAs, GaSb, InP, InAs, InSb,AlGaP, AlLnP, AlGaAs, AlInAs, AlAsSb, GaInAs, GaInSb, GaAsSb, andInAsSb; a II-VI compound semiconductor such as ZnS, ZnSe, ZnTe, CaSe,CdTe, HgSe, HgTe, and CdS; and a metal such as Pd, Pt, Al, Au, and Agcan be used as the conductive or semiconductive material.

An oxide such as SrTiO₃, BaTiO₃, BaZrO₃, LaAlO₃, ZrO₂, 8% Y₂O₃—ZrO₂,MgO, MgAl₂O₄, LiNbO₃, LiTaO₃, Al₂O₃, and ZnO; a single semiconductorsuch as Si, Ge, and diamond; a III-V compound semiconductor such asAlAs, AlSb, AlP, GaAs, GaSb, InP, InAs, InSb, AlGaP, AlLnP, AlGaAs,AlInAs, AlAsSb, GaInAs, GaInSb, GaAsSb, and InAsSb; and a II-VI compoundsemiconductor such as ZnS, ZnSe, ZnTe, CaSe, Cdte, HgSe, HgTe, and CdScan be used as the substrate material when the epitaxial thin film orthe conductive or semiconductive thin film having a single orientationis provided on the surface thereof. When the conductive orsemiconductive thin film is provided on the non-oxide substrate, it iseffective that an oxide such as SrTiO₃, BaTiO₃, BaZrO₃, LaAlO₃, ZrO₂, 8%Y₂O₃—ZrO₂, MgO, MgAl₂O₄, LiNbO₃, LiTaO₃, Al₂O₃, and ZnO is formed as thebuffer layer on the surface of the non-oxide substrate, and thereafter,the conductive or semiconductive thin film is formed thereon.

The material which has the refractive index lower than that of theoptical waveguide layer 16 and the relative dielectric constant not lessthan 8 is used as the buffer layer 14. The ratio of the relativedielectric constant of the buffer layer 14 to the relative dielectricconstant of the optical waveguide layer 16 is preferably not less than0.002, and the ratio is more preferably not less than 0.006. It isnecessary that the material of the buffer layer 14 holds an epitaxialrelationship with the materials of the conductive substrate 12 and theoptical waveguide layer 16. A similar crystal structure and a differencein lattice constant of not more than 10% can be cited as the conditionthat the epitaxial relationship can be held. However, as long as theepitaxial relationship can be held, it is not always necessary tosatisfy the condition.

Specifically, in a perovskite type of oxide of an ABO₃ type, SrTiO₃,BaTiO₃, (Sr_(1-x)Ba_(x))TiO₃ (0<x<1.0), PbTiO₃,Pb_(1-x)La_(x)(Zr_(y)Ti_(1-y))_(1-x/4)O₃ (0<x<0.3, 0<y<1.0, and PZT,PLT, and PLZT depending on values of x and y), Pb(Mg_(1/3)Nb_(2/3))O₃,and KNbO₃ can be cited as a tetragonal system, a trigonal system, anorthorhombic system, or a quasi-cubic system, and a ferroelectricsubstance such as LiNbO₃ and LiTaO₃ can be cited as a hexagonal system.In a tungsten bronze type of oxide, Bi₄Ti₃O₁₂, Pb₂KNb₅O₁₅, K₃Li₂Nb₅O₁₅,and ZnO and substituted derivatives of these compounds can be cited inaddition to Sr_(x)Ba_(1-x)Nb₂O₆ and Pb_(x)Ba_(1-x)Nb₂O₆.

It is preferable that the film thickness of the buffer layer 14 is notless than 10 nm. As described above, the film thickness ratio of thebuffer layer 14 to the optical waveguide layer 16 is preferably not lessthan 0.1, and the ratio is more preferably not less than 0.5.

The oxide having a refractive index larger than that of the buffer layer14 is used for the optical waveguide layer 16. Specifically, in theperovskite type of oxide of the ABO₃ type, BaTiO₃, PbTiO₃,Pb_(1-x)La_(x)(Zr_(y)Ti_(1-y))_(1-x/4)O₃ (PZT, PLT, and PLZT dependingon values of x and y), Pb(Mg_(1/3)Nb_(2/3))O₃, and KNbO₃ can be cited asthe tetragonal system, the trigonal system, the orthorhombic system, orthe quasi-cubic system, and the ferroelectric substance such as LiNbO₃and LiTaO₃ can be cited as the hexagonal system. In the tungsten bronzetype of oxide, Sr_(x)Ba_(1-x)Nb₂O₆ and Pb_(x)Ba_(1-x)Nb₂O₆ can be cited.Bi₄Ti₃O₁₂, Pb₂KNb₅O₁₅, and K₃Li₂Nb₅O₁₅ and substituted derivatives ofthese compounds can be cited in addition to the above materials.

Usually the film thickness of the optical waveguide layer 16 is set inthe range from 0.1 μm to 10 μm, and the film thickness of the opticalwaveguide layer 16 can be appropriately selected in accordance with thepurpose.

When a cladding layer is provided, the material similar to the bufferlayer 14 can be used as the cladding layer. For the material of thecladding layer, it is not always necessary to hold an epitaxialrelationship with the optical waveguide layer, and it is possible to usea polycrystalline thin film. When a uniform interface is required, it isnecessary to hold the epitaxial relationship with the optical waveguidelayer. A similar crystal structure and a difference in lattice constantof not more than 10% can be cited as the condition that the epitaxialrelationship can be held. However, as long as the epitaxial relationshipcan be held, it is not always necessary to satisfy the condition.

Specifically, in the perovskite type of oxide of the ABO₃ type, SrTiO₃,BaTiO₃, (Sr_(1-x)Ba_(x))TiO₃, PbTiO₃,Pb_(1-x)La_(x)(Zr_(y)Ti_(1-y))_(1-x/4)O₃, Pb(Mg_(1/3)Nb_(2/3))O₃, andKNbO₃ can be cited as the tetragonal system, the trigonal system, theorthorhombic system, or the quasi-cubic system, and the ferroelectricsubstance such as LiNbO₃ and LiTaO₃ can be cited as the hexagonalsystem. In the tungsten bronze type of oxide, Sr_(x)Ba_(1-x)Nb₂O₆ andPb_(x)Ba_(1-x)Nb₂O₆ can be cited. Bi₄Ti₃O₁₂, Pb₂KNb₅O₁₅, K₃Li₂Nb₅O₁₅,and ZnO and substituted derivatives of these compounds can be cited inaddition to the above materials.

It is preferable that the film thickness of the cladding layer is notless than 10 nm. The film thickness ratio of the cladding layer to theoptical waveguide layer 16 is preferably not less than 0.1, and theratio is more preferably not less than 0.5.

The various combinations satisfying the above-described conditions canbe adopted as the combination of the materials of the conductivesubstrate, the buffer layer, the optical waveguide layer, and thecladding layer. It is preferable to use a doped SrTiO₃ single crystalsemiconductor substrate, it is preferable that a doped SrTiO₃semiconductor thin film is used as the conductive substrate, or it ispreferable to use the substrate in which a SrRuO₃ conductive thin filmis grown after an MgO buffer layer is grown on the Si single crystal.When the conductive substrate is used, it is most effective thatPb_(1-x)La_(x)(Zr_(y)Ti_(1-y))_(1-x/4)O₃ (PLZT) is used for each of thebuffer layer, the optical waveguide layer, and the cladding layer. PLZThas a perovskite structure similar to the above-described conductivesubstrate, so that the difference in lattice constant is small and goodepitaxial growth is possible. PLZT has the refractive index larger thanthe refractive index of 2.399 of the conductive substrate, and PLZT hasa higher electro-optic coefficient. Further, the refractive index ofeach layer can be largely changed by changing the composition, i.e., theratio of Pb, La, Zr, and Ti.

The metal such as Al, Ti, Cr, Ni, Cu, Pd, Ag, In, Sn, Ta, W, Ir, Pt, andAu and the alloy of these metals, and the oxide such as Al-doped ZnO,In₂O₃, ITO, RuO₂, BaPbO₃, SrRuO₃, YBa₂Cu₃O_(7-x), SrVO₃, LaNiO₃,La_(0.5)Sr_(0.5)CoO₃, ZnGa₂O₄, CdGa₂O₄, CdGa₂O₄, Mg₂TiO₄, and MgTi₂O₄can be used as the upper electrode 26. When a cladding layer is used, itis desirable to use a metal electrode in which micropatterning can beeasily performed. When a cladding layer is not used, it is effective touse an oxide electrode, and it is desirable to use a transparent oxideelectrode such as ITO. When fatigue or DC drift is generated withincreasing operating time, it is effective to use an oxide electrode.

Each of the cladding layer, the optical waveguide layer, and the bufferlayer can be produced by any one of a vapor phase epitaxial growthmethod such as electron-beam evaporation, flash evaporation, ionplating, RF-magnetron sputtering, ion-beam sputtering, laser abrasion,MBE, CVD, plasma CVD, and MO-CVD; a solid phase epitaxial growth methodwhich heats an amorphous thin film formed by the above-described vaporphase growth; and a solid phase epitaxial growth method which heats theamorphous thin film produced by a wet process such as a sol-gel processand an MOD method.

Among the epitaxial growth methods, the solid phase epitaxial growthmethod is preferable from the viewpoint of waveguide quality andwaveguide patterning. The solid phase epitaxial growth method caninclude an applying process for applying to the substrate a solution ofa metal organic compound such as metal alkoxide and organometallic saltby the wet process such as the sol-gel process and the MOD method; aprocess for forming the amorphous phase by heating; and a process forperforming crystallization by heating. In the solid phase epitaxialgrowth method described above, facilities cost becomes lower whencompared with each of the vapor phase epitaxial growth methods, gooduniformity is obtained in a substrate plane, and it is easy to controlthe refractive index which is important for structure control of thebuffer layer, optical waveguide layer, and the cladding layer, so thatthe optical switch can be produced with good reproducibility. Therefore,in the solid phase epitaxial growth method, it is possible to grow thebuffer layer and the like in which light propagation loss is low. Thesolid phase epitaxial growth method is also most suitable for thepatterning because the solid phase epitaxial growth method includes theprocess for forming an amorphous thin film.

Since the optical switch is produced by the epitaxial growth of eachlayer, when compared with the conventional material and structure, it iseasy to control the refractive index and thickness of each of thesubstrate, the buffer layer, the thin film optical waveguide, and thecladding layer. For example, when Pb(Zr_(x)Ti_(1-x))O₃ (0<x<1.0) is usedfor the buffer layer, the thin film optical waveguide, and the claddinglayer, not only mutual crystal conformation is good, but also therefractive index for the wavelength of 0.633 μm can be selected from awide range of about 2.45 to about 2.70 in accordance with thecomposition. Further, the channel waveguides having various shapes canbe easily produced by producing each layer by the thin film growthprocess. Therefore, the effective refractive index of the channeloptical waveguide or the difference in refractive index between thechannel optical waveguide and the outside can be designed over the widerange, and a curvature of a curved channel can be increased if necessarywhile the radiation loss is suppressed, so that a large-scale matrixoptical switch can be easily produced.

(Modifications)

In the above-described embodiment, it is described that the channeloptical waveguide having a ridge structure in which a convex portion isprovided on the optical waveguide layer is formed. However, any one ofthe embedded type, the ridge type, and the rib type which are usuallyadopted can be used for the channel optical waveguide. When a thin filmis laminated by epitaxial growth, it is possible to easily produce theridge structure in which a convex portion is provided on the opticalwaveguide layer, the embedded structure in which a convex portion isprovided on the optical waveguide layer and then the convex portion iscoated by the cladding layer, and the rib structure in which the opticalwaveguide layer is provided after a concave portion is provided in thebuffer layer.

The inventors have invented the structure as follows (Japanese PatentLaid-Open No. 2000-047272). An epitaxial or single-orientated bufferlayer is provided on a conductive or semiconductive single crystalsubstrate which becomes a lower electrode. On the buffer layer, anepitaxial or single-orientated oxide thin film optical waveguide whichhas the refractive index larger than that of the buffer layer and anelectro-optic effect is provided. On the optical waveguide a claddinglayer which has the refractive index smaller than that of the opticalwaveguide and the high dielectric constant is provided if necessary, anthe upper electrode is provided on the cladding layer. Therefore, evenin the case of an oxide ferroelectric material, the structure in whichthe optical waveguide is sandwiched between the upper and lowerelectrodes can be formed, and the low voltage can induce a large changein refractive index without losing the low light-propagation-losscharacteristics.

In accordance with the study of the inventors, as described above, theepitaxial or single-orientated buffer layer is provided on theconductive or semiconductive single crystal substrate which becomes thelower electrode, the epitaxial or single-orientated oxide thin filmoptical waveguide which has the refractive index larger than that of thebuffer layer and the electro-optic effect is provided on the bufferlayer, and the upper electrode is provided on the optical waveguide.Therefore, the change in refractive index can be effectively performedby applying the voltage to obtain a clear refractive index contrastnecessary for the total reflection type of optical switch. As a result,the structure described above is extremely effective for decreasingdrive voltage, and the length of each electrode can be significantlyreduced.

Further, since the optical switch is produced by the epitaxial growth ofeach layer, when compared with the conventional material and structure,it is easy to control the refractive index and thickness of each of thesubstrate, the buffer layer, the thin film optical waveguide, and thecladding layer. For example, when Pb(Zr_(x)Ti_(1-x))O₃ (0<x<1.0) is usedfor the buffer layer, the thin film optical waveguide, and the claddinglayer, not only mutual crystal conformation is good, but also therefractive index for the wavelength of 0.633 μm can be selected from awide range of about 2.45 to about 2.70 in accordance with thecomposition. Further, the channel waveguides having various shapes canbe easily produced by producing each layer by the thin film growthprocess. Therefore, the effective refractive index of the channeloptical waveguide or the difference in refractive index between thechannel optical waveguide and the outside can be designed over the widerange, and the curvature of the curved channel can be increased ifnecessary while the radiation loss is suppressed, so that a large-scalematrix optical switch can be easily produced.

As a result of diligent investigation of various structures of an N×Mmatrix optical switch, an end face for incidence from N optical fibersand an end face for outgoing to the optical fiber are provided at bothends of the single crystal substrate, wiring between the incident endface and the outgoing end face is performed with the channel opticalwaveguide and the curved optical waveguide which constitute the totalreflection type of optical switch, and the light beam incident from eachoptical fiber through the incident end face is switched by applying thevoltage between the upper and lower electrodes which are provided on thecrossover portion of the total reflection type of switch. Therefore, theoptical path can be switched to the desired M optical fibers, theswitching between ports with a proper interval can be realized with lowdrive voltage, and the electrode length and the length of the curvedchannel optical waveguide can be also reduced if necessary, so that themultiplicity of optical switches can be integrated in the substratewafer having the same size as the conventional one to obtain alarge-scale matrix optical switch.

EXAMPLES

Hereinafter, Examples of the invention will be described in detail.

EXAMPLE 1

The specific Example of the 1×2 optical switch of the Y-branching typeabove described will be described below.

As shown in FIG. 5, the PLZT buffer layer 14 having the refractive indexof 2.410 at the wavelength of 1.55 μm is formed by the solid phaseepitaxial growth on the conductive substrate 12 made of Nb-doped SrTiO₃(100) single crystal semiconductor so that the thickness of the bufferlayer 14 becomes 2000 nm, and then the PLZT optical waveguide layer 16having the refractive index of 2.446 is formed by the solid phaseepitaxial growth so that the thickness of the optical waveguide layer 16becomes 2500 nm. The difference in refractive index between the bufferlayer 14 and the optical waveguide layer 16 is 0.4%.

The solid phase epitaxial growth will be described in detail below. Leadacetate anhydride Pb(CH₃COO)₂, lanthanum isopropoxide La(O-i-C₃H₇)₃,zirconium isopropoxide Zr(O-i-C₃H₇)₄, and titanium isopropoxideTi(O-i-C₃H₇)₄ are used as a starting material, the starting material issolved in 2-methoxy ethanol, distillation and reflux are performed, andfinally PLZT buffer layer precursor solution having concentration of0.6M in terms of Pb concentration is obtained.

Then, spin coating of the precursor solution is performed onto thesubstrate made of Nb-doped SrTiO₃ (100) single crystal, in whichcleaning, etching, and drying are already performed. The substrate isheated in O₂ atmosphere and held at 350° C. After the substrate isfurther held at 750° C., the substrate is cooled. The solid phaseepitaxial growth of the PLZT buffer layer 14 is performed by repeatingthe above process. Similarly, the solid phase epitaxial growth of thePLZT optical waveguide layer 16 is performed.

Then, an electrode pattern is formed by photolithography. Afterdepositing on the PLZT optical waveguide layer 16 a laminated thin filmin which an ITO thin film having the thickness of 300 nm and the Au thinfilm having the thickness of 200 nm are laminated, the patterns of theupper electrodes 26A, 26B, and 26C shown in FIG. 1 are formed by alift-off method.

Further, the pattern of the channel waveguide 24C shown in FIG. 1 isformed by photolithography, and the channel waveguide 24 having theridge structure is formed by ICP dry etching. The width of the channelwaveguide is 4 μm, the taper length is 700 μm, the maximum width of thetaper portion is 20 μm, the length of the linear portion is 400 μm, andthe Y-crossing angle is 1.0°.

Finally, the substrate is cut into optical switch chips having anoverall length of about 6 mm by dicing. The incident end face and theoutgoing end face are formed by polishing.

A crystallographic relationship of each layer of the obtained opticalswitch 10 is as follows. The single-orientated PLZT (100) thin filmoptical waveguide//PLZT (100) buffer layer//Nb-doped SrTiO₃ (100)conductive substrate and in-plane orientation PLZT [001] opticalwaveguide layer//PLZT [001] buffer layer//Nb-doped SrTiO₃ [001]conductive substrate.

The electro-optic coefficient r of 85 pm/V is obtained for the PLZToptical waveguide layer, after initialization voltage, i.e., polingvoltage of 50V is applied between the lower electrode of the Nb-dopedSrTiO₃ substrate and the upper electrode.

Single mode optical fibers are disposed at the incident end face and theoutgoing end face of the 1×2 optical switch of Example 1. A laser beamhaving the wavelength of 1.55 μm is introduced from the optical fiber tothe incident port 18, and voltage is applied between the conductivesubstrate 12 and the upper electrodes 26B and 26C. Accordingly, therefractive index of the optical waveguide layer 16 between theelectrodes is decreased, the laser beam introduced from the incidentport 18 is totally reflected from the Y-crossover portion 30 to selectthe outgoing port 20, and switching of the optical fiber path isperformed as a digital type of switch. When voltage is applied betweenthe conductive substrate 12 and the upper electrodes 26A and 26C, theoutgoing port 22 is selected, and switching of the optical fiber path issimilarly performed as a digital type of switch.

FIG. 9 shows a relationship between the drive voltage and the crosstalk(difference in light quantity between the outgoing ports) which areobtained by this optical switch. As can be seen from FIG. 9, thecrosstalk becomes 20 dB at voltage 8V, and the digital characteristiccan be obtained that the crosstalk not less than 20 dB is maintained aslong as the voltage not less than 8V is applied.

As described above, in the produced 1×2 optical switch 10, in spite ofthe fact that the overall length is as small as about 6 mm, the drivevoltage becomes 8V, which is about a fraction of the voltage requiredfor the conventional 1×2 optical switch made of LiNbO₃. The switchingspeed is 3 ns, the crosstalk is not more than 20 dB, and the insertionloss is not more than 3 dB. The 1×2 optical switch which is independentof polarized wave and has good characteristics is obtained.

EXAMPLE 2

In Example 2, the difference in refractive index between the bufferlayer 14 and the channel optical waveguide 24 is set to 0.3%. The widthof the channel waveguide is set to 4 μm, the taper length is set to9001m, the maximum width of the taper portion is set to 24 μm, thelength of the linear portion is set to 400 μm, and the Y-crossing angleis set to 0.5°. As shown in FIG. 11, the groove portion 46 is formedinstead of the formation of the upper electrode 26C, and the patterns ofupper electrodes 26D and 26E are formed so that the channel waveguide 24is substantially covered with the patterns. Except for the above, a 1×2optical switch 60 is formed in a manner similar to Example 1.

When the optical switch 60 is evaluated in the manner similar to Example1, the relationship between the drive voltage and the crosstalk isobtained as shown in FIG. 12. As can be seen from FIG. 12, the crosstalkbecomes 20 dB at voltage 5V, and the digital characteristic can beobtained that the crosstalk not less than 20 dB is maintained as long asthe voltage not less than 5V is applied.

As described above, in the produced 1×2 optical switch 60, in spite ofthe fact that the overall length is as small as about 6 mm, the drivevoltage becomes 5V, which is in the range where a CMOS can be driven andwhich is one-tenth of the voltage required for the conventional 1×2optical switch made of LiNbO₃. The switching speed is 4 ns, thecrosstalk is not more than 20 dB, and the insertion loss is not morethan 3 dB. The 1×2 optical switch which is independent of polarized waveand has good characteristics is obtained.

When a semiconductor waveguide material such as InGaP is used, theoptical switch having the plane structure substantially similar to FIG.11 can be produced. In the case of using a substrate in which a channelwaveguide is formed by diffusing Ti in a LiNbO₃ single crystal wafer,the patterns of upper electrodes 26H and 26I of FIG. 17 can be formed asa co-planar type of electrode pattern.

EXAMPLE 3

In Example 3, the difference in refractive index between the bufferlayer 14 and the channel optical waveguide 24 is set to 0.2%, the widthof the channel waveguide is set to 4 μm, the taper length is set to 700μm, the maximum width of the taper portion is set to 24 μm, the lengthof the linear portion is set to 400 μm, and the Y-crossing angle is setto 0.5°. Except for the above, the 1×2 optical switch 60 in which thepatterns of the upper electrodes 26D and 26E are formed is formed in themanner similar to Example 2.

When the optical switch 60 is evaluated in the manner similar to Example1, the relationship between the drive voltage and the crosstalk isobtained as shown in FIG. 13. As can be seen from FIG. 13, the crosstalkbecomes 20 dB at voltage 9V, and the digital characteristic can beobtained that the crosstalk not less than 20 dB is maintained as long asthe voltage not less than 9V is applied.

As described above, in the produced 1×2 optical switch 60, in spite ofthe fact that the overall length is as small as about 6 mm, the drivevoltage becomes 9V which is about a fraction of the voltage required forthe conventional 1×2 optical switch made of LiNbO₃. The switching speedis 3 ns, the crosstalk is not more than 20 dB, and the insertion loss isnot more than 3 dB. The 1×2 optical switch which is independent ofpolarized wave and has good characteristics is obtained.

EXAMPLE 4

In Example 4, as shown in FIG. 14, plural 1×2 optical switches ofExample 2 are arrayed on the same substrate 52 by the combination of the1×2 optical switches to form a 1×8 optical switch 70 in which a channelwaveguide 54 having plural branches is formed. One single mode opticalfiber is disposed at the incident end face of the optical switch 70 andeight single mode optical fibers are disposed at the outgoing end faceat intervals of 254 μm.

A laser beam having the wavelength of 1.55 μm is introduced to theincident port of the 1×8 optical switch of Example 4 from the opticalfiber, and the optical fiber path of the laser beam introduced from theincident port is switched in the form of a digital type of opticalswitch by applying voltage 8V between the conductive substrate 12 andthe upper electrode 26D.

When the 1×8 optical switch 70 is evaluated in the manner similar toExample 1, in the produced 1×8 optical switch 70, in spite of the factthat the overall length is as small as about 20 mm, the drive voltagebecomes 5V, which is in the range where a CMOS can be driven and whichis about one-tenth of the voltage required for the conventional 1×8optical switch made of LiNbO₃. The switching speed is 4 ns, thecrosstalk becomes not more than 20 dB, and the insertion loss is notmore than 5 dB. The 1×8 optical switch which is independent of polarizedwave and has good characteristics is obtained.

EXAMPLE 5

In Example 5, a digital type and a strictly nonblocking type of 8×8optical switch 80 is formed by arraying one hundred twelve 1×2 opticalswitches of Example 2 on the same substrate. A digital type and astrictly nonblocking type of 8×8 optical switch is formed as anotherexample of the invention by combining one hundred twelve 1×2 opticalswitches having the same configuration as that of Example 2. Theincident end face, the outgoing end face, and the 1×2 optical switchesare connected to one another with an S-shaped type, linear type, andX-shaped type of channel optical waveguide. Single mode optical fiberarrays in which eight optical fibers are provided at intervals of 127 μmare disposed at the incident end face and the outgoing end face,respectively.

When the produced 8×8 optical switch is evaluated in the manner similarto Example 1, in the 8×8 optical switch, in spite of the fact that theoverall length is as small as about 30 mm, the drive voltage becomes 5Vwhich is about one-tenth of the voltage required for the conventional8×8 optical switch made of LiNbO₃. The switching speed is 4 ns, thecrosstalk becomes not more than 40 dB because the switch is formed intwo stages, and the insertion loss is not more than 7 dB. The 8×8optical switch which is independent of polarized wave and has goodcharacteristics is obtained.

EXAMPLE 6

In Example 6, the epitaxial MgO (100) film 17 is grown on a Si (100)single crystal substrate, and then the SrRuO₃ (100) conductive thin film19 is grown as a lower electrode, and thereafter the PLZT buffer layer14 and the PLZT optical waveguide layer 16 are formed. The difference inrefractive index between the buffer layer 14 and the channel opticalwaveguide 24 is set to 0.2%, the width of the channel waveguide is setto 4 μm, the taper length is set to 600 μm, the maximum width of thetaper portion is set to 24 μm, the length of the linear portion is setto 400 μm, and the Y-crossing angle is set to 0.5°. As shown in FIGS. 15and 16, the groove portion 46 is formed instead of the formation of theupper electrode 26C, and the patterns of upper electrodes 26F and 26Gwhose electrode lengths are shortened for the purpose of high speed areformed. A polymer layer 56 is formed as a cladding layer on the opticalwaveguide layer 16. Except for the above, a 1×2 optical switch 80 isformed in the manner similar to Example 2.

When the produced 1×2 optical switch 80 is evaluated in the mannersimilar to Example 1, in the 1×2 optical switch 80, in spite of the factthat the overall length is as small as about 6 mm, the digitalcharacteristics that the drive voltage becomes 12V and the crosstalk is20 dB are obtained, the switching speed is 2 ns, and the insertion lossis not more than 3 dB. The 1×2 optical switch which is independent ofpolarized wave and has good characteristics is obtained.

In the optical switch of the present invention, it is preferable thatthe crossing angle ranges from 0.5° to 2.0°. Since the reflection planeis formed near the crossover portion located on the upstream side of thebranching portion in the light propagation direction, the reflectionplane is hardly affected by the shape of the branching portion of thechannel waveguide or the production error of the electrode, andcrosstalk is decreased when compared with the conventional Y-branchingoptical switch.

It is preferable that the control electrode is formed on the opticalwaveguide layer so that the angle formed by the centerline of theincident-side channel waveguide and the edge of the control electrode onthe waveguide side becomes equal to the angle formed by the centerlineof the outgoing-side channel waveguide and the edge of the controlelectrode on the waveguide side, and the incident light signal istotally reflected from the reflection plane.

It is preferable that the taper portion extending toward the propagationdirection of the light signal is formed on the outgoing-side of theincident-side channel waveguide and the taper portion is connected tothe incident-side of the outgoing-side channel waveguide through thecoupling portion. Crosstalk can be further decreased by forming thecrossover portion in the tapered shape. It is preferable that thereverse taper portion extending toward the opposite direction to thepropagation direction of the light signal is formed on the incident-sideof the outgoing-side channel waveguide and the coupling portion isformed by the linear channel waveguide having the same width as that ofthe outgoing end of the incident-side channel waveguide.

In the optical waveguide layer, it is also possible that the groove isformed between the outgoing-side channel waveguides adjacent to eachother. When the groove is formed, the control electrode forms thereflection plane which is contiguous to the interface between theoptical waveguide layer and the groove.

1. An optical switch comprising: a substrate which has at leastconductivity; an optical waveguide layer which is formed on thesubstrate, the optical waveguide layer including an incident-sidechannel waveguide on which a light signal is incident and a plurality ofoutgoing-side channel waveguides branching from the incident-sidechannel waveguide; and a control electrode which is formed on theoptical waveguide layer, the control electrode forming, near a crossoverportion of the plurality of outgoing-side channel waveguides, areflection plane which reflects the incident light signal by applyingwith the substrate a voltage to the optical waveguide layer to controlthe refractive index of the optical waveguide layer, and the controlelectrode switching propagation paths of the light signal.
 2. An opticalswitch according to claim 1, wherein a grooved portion is formed betweenadjacent outgoing-side channel waveguides in the optical waveguidelayer.
 3. An optical switch according to claim 1, wherein the substratehas semiconductivity.
 4. An optical switch according to claim 1, whereinthe control electrode is formed on the optical waveguide layer so thatan angle formed by a centerline of the incident-side channel waveguideand an edge on a waveguide-side of the control electrode becomes equalto an angle formed by a centerline of the outgoing-side channelwaveguide and the edge on the waveguide-side of the control electrode.5. An optical switch according to claim 1, wherein the angle formed bytwo centerlines of adjacent outgoing-side channel waveguides is withinthe range from 0.5° to 2.0°.
 6. An optical switch according to claim 1,wherein a taper portion which broadens out in the propagation directionof the light signal is formed on the outgoing-side of the incident-sidechannel waveguide, and the taper portion is connected to theincident-side of the outgoing-side channel waveguide through a couplingportion.
 7. An optical switch according to claim 6, wherein a reversetaper portion which narrows in the propagation direction of the lightsignal is formed on the incident-side of the outgoing-side channelwaveguide.
 8. An optical switch according to claim 6, wherein thecoupling portion is formed by a linear channel waveguide having the samewidth as the outgoing-side end of the incident-side channel waveguide.9. An optical switch according to claim 6, wherein a taper length of thetaper portion is within the range from 200 μm to 2000 μm.
 10. An opticalswitch according to claim 6, wherein the maximum width of the taperportion is within the range from 5 μm to 50 μm.
 11. An optical switchaccording to claim 7, wherein a taper length of the reverse taperportion is within the range from 200 μm to 2000 μm.
 12. An opticalswitch according to claim 7, wherein the maximum width of the reversetaper portion is within the range from 5 μm to 50 μm.
 13. An opticalswitch according to claim 1, wherein the substrate includes a singlecrystal substrate portion, and the optical waveguide layer comprises aferroelectric oxide substance.
 14. An optical switch according to claim13, wherein the single crystal substrate portion has at leastconductivity.
 15. An optical switch according to claim 13, wherein thesubstrate has a thin film at least having conductivity provided on asurface of the single crystal substrate portion.
 16. An optical switchaccording to claim 15, wherein the thin film has semiconductivity. 17.An optical switch according to claim 13, wherein the optical waveguidelayer comprises an epitaxial ferroelectric oxide substance.
 18. Anoptical switch according to claim 13, wherein the optical waveguidelayer comprises a ferroelectric oxide substance having a singleorientation.
 19. An optical switch according to claim 13, wherein abuffer layer having a refractive index smaller than that of the opticalwaveguide layer is provided between the substrate and the opticalwaveguide layer.
 20. An optical switch according to claim 19, whereinthe buffer layer comprises an epitaxial oxide.
 21. An optical switchaccording to claim 19, wherein the buffer layer comprises an oxidehaving a single orientation.
 22. An optical switch according to claim13, wherein a cladding layer comprising an oxide which has a refractiveindex smaller than that of the optical waveguide layer is providedbetween the optical waveguide layer and the control electrode.
 23. Anoptical switch according to claim 13, wherein the optical waveguidelayer comprises Pb_(1-x)La_(x)(Zr_(y)Ti_(1-y))_(1-x/4)O₃ (0<x<0.3 and0<y<1.0).
 24. An optical switch according to claim 19, wherein thebuffer layer comprises Pb_(1-x)La_(x)(Zr_(y)Ti_(1-y))_(1-x/4)O₃ (0<x<0.3and 0<y<1.0).
 25. An optical switch according to claim 22, wherein thecladding layer comprises Pb_(1-x)La_(x)(Zr_(y)Ti_(1-y))_(1-x/4)O₃(0<x<0.3 and 0<y<1.0).
 26. An optical switch according to claim 14,wherein the single crystal substrate portion includes SrTiO₃ doped withimpurity elements.
 27. An optical switch according to claim 15, whereinthe single crystal substrate portion comprises Si and has an oxidebuffer layer on the surface.
 28. An optical switch according to claim15, wherein the thin film is an epitaxial thin film.
 29. An opticalswitch according to claim 15, wherein the thin film is a thin filmhaving a single orientation.
 30. An optical switch according to claim 1,wherein the incident-side channel waveguide and the outgoing-sidechannel waveguides are embedded channel waveguides.
 31. An opticalswitch according to claim 1, wherein the incident-side channel waveguideand the outgoing-side channel waveguides are ridge channel waveguides.32. An optical switch comprising: a substrate which has at leastconductivity; an optical waveguide layer which is formed on thesubstrate, the optical waveguide layer including an incident-sidechannel waveguide on which a light signal is incident and a plurality ofoutgoing-side channel waveguides branching from the incident-sidechannel waveguide, the optical waveguide layer having a grooved portionformed between adjacent outgoing-side channel waveguides; and a controlelectrode which is formed on the optical waveguide layer, the controlelectrode forming, near a crossover portion of the plurality ofoutgoing-side channel waveguides, a reflection plane which reflects theincident light signal by applying a voltage with the substrate to theoptical waveguide layer to control the refractive index of the opticalwaveguide layer, the reflection plane being contiguous to an interfacebetween the optical waveguide layer and the grooved portion, and thecontrol electrode switching propagation paths of the light signal.
 33. Amatrix optical switch comprising: a substrate which has at leastconductivity; and a plurality of optical switch units which are arrangedin a matrix on the substrate, wherein each of the plurality of opticalswitch units includes an optical waveguide layer which is formed on thesubstrate, the optical waveguide layer including an incident-sidechannel waveguide on which a light signal is incident and a plurality ofoutgoing-side channel waveguides branching from the incident-sidechannel waveguide, and a control electrode which is formed on theoptical waveguide layer, the control electrode forming, near a crossoverportion of the plurality of outgoing-side channel waveguides, areflection plane which reflects the incident light signal by applying avoltage with the substrate to the optical waveguide layer to control therefractive index of the optical waveguide layer, and the controlelectrode switching propagation paths of the light signal.
 34. A matrixoptical switch according to claim 33, wherein the substrate hassemiconductivity.
 35. A matrix optical switch comprising: a substratewhich has at least conductivity; and a plurality of optical switch unitswhich are arranged in a matrix on the substrate, wherein each of theplurality of optical switch units includes an optical waveguide layerwhich is formed on the substrate, the optical waveguide layer includingan incident-side channel waveguide on which a light signal is incidentand a plurality of outgoing-side channel waveguides branching from theincident-side channel waveguide, the optical waveguide layer having agrooved portion formed between adjacent outgoing-side channelwaveguides, and a control electrode which is formed on the opticalwaveguide layer, the control electrode forming, near a crossover portionof the plurality of outgoing-side channel waveguides, a reflection planereflecting the incident light signal by applying with the substrate avoltage to the optical waveguide layer to control the refractive indexof the optical waveguide layer, the reflection plane being contiguous toan interface between the optical waveguide layer and the groovedportion, and the control electrode switching propagation paths of thelight signal.
 36. A matrix optical switch according to claim 35, whereinthe substrate has semiconductivity.